The present invention relates to an output control device for a high-frequency power-supply unit capable of controlling power output from the power-supply unit which is used in a power-supply circuit loaded with a plasma generator, an induction heater, and a resonance circuit for example.
Conventionally, there is a semiconductor inverter which is used as a circuit for generating high-freqency power to allow a plurality of arms to respectively perform push-and-pull operations and alternately turn themselves ON and OFF before eventually converting the DC power into a specific high-frequency power.
Of a variety of conventional systems made available for controlling power output from the semiconductor inverter, there is a system for varying either voltage or current of power supplied to the semiconductive inverter by using a thyristor.
However, this system involves a complicated constitution which causes the power factor to eventually be lowered. In addition, due to slow speed in responding to variable load, elements may easily be destroyed by overcurrent when load is shorted or by overvoltage when load is freed.
Furthermore, there is another conventional system used for controlling power output from the semiconductive inverter, in which, if a resonance circuit makes up the load, frequencies output from the inverter are variably controlled.
When frequency output from the semiconductive inverter is identical to the resonance frequency output from the resonance circuit which substantially makes up load, the system designates this frequency to be the maximum output, i.e., the serial resonance. Based on this, the system variably controls the output from the semiconductive inverter by either raising the frequency output from this inverter to a specific level higher than the resonance frequency or conversely lowering it to a specific level lower than the resonance frequency.
Nevertheless, when the system variably controls the frequency output from the inverter to a level lower than the resonance frequency, a state of short-circuit can easily take place with the power during the recovery period of a fly-wheel diode that substantially makes up the inverter. Thus, conventionally, the output frequency is set to a level higher than the resonance frequency.
As a result, the system unavoidably needs to hold the resonance frequency of the resonance circuit at a specific level lower than the maximum operative frequency of the semiconductive inverter, thus operation of the resonance circuit making up the load can be constrained by operation of the semiconductive inverter itself.
Furthermore, variable characteristic of the power output from the resonance circuit making up the load is adversely affected by Q which is the aimed acuteness of resonance of this resonance circuit. In particular, when a lesser value of Q is present, even when raising the level of frequency output from the inverter, output power cannot fully be lowered. Conversely, when a greater value of Q is present, power output from the inverter can significantly be varied by even the slightest variation of the frequency, thus making it difficult for any of these conventional systems to correctly control the output power.